Digital voltmeter topology

ABSTRACT

A system may include two input terminals, e.g., HI and LO, and a floating circuit that is physically separate from the input terminals and includes a gain amplifier. The floating circuit can be surrounded by a conductive enclosure that is electrically connected to the second input terminal. The floating circuit can further switch between input signals received from the first and second input terminals to the gain amplifier and the floating circuit ground.

BACKGROUND

Over the years, certain digital voltmeter (DVM) designs have improvedinput signal observation time but have also concurrently introducedsensitivity to the offset drift of the input amplifier feedback, thuslimiting the performance of the product. For example, certain nanoVoltDVMs have used a measure zero cycle to remove the input amplifier'soffset. In such DVMs, a measurement with the input applied to the inputamplifier is typically made followed by a measurement with zero applied.The difference thus removes the input amplifier's offset but the zerocycle is time spent not measuring the input signal.

Other designs have used a reversing input amplifier that inverses itsoffset between each measurement. Thus, by averaging two adjacentmeasurements, the input amplifier's offset is cancelled and the inputcan be observed during both measurements thereby reducing the noise.However, the offset drift of the input amplifier's feedback is notcancelled and, therefore, limits the performance of the unit.

Accordingly, a need remains for a design that maintains high observationtime while removing the detrimental sensitivity to the feedback offsetdrift.

SUMMARY

Embodiments of the disclosed technology are generally directed tolow-voltage voltage measurement devices that include two inputs (HI andLO) and a floating digital voltmeter (DVM) within a guard shield.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a digital voltmeter (DVM) circuittopology in accordance with certain embodiments of the disclosedtechnology.

DETAILED DESCRIPTION

Embodiments of the disclosed technology are generally directed to atopology that has good observation efficiency and cancels the inputamplifier's offset as well as its feedback offset.

FIG. 1 illustrates an example of a digital voltmeter (DVM) circuittopology 100 in accordance with certain embodiments of the disclosedtechnology. The DVM circuit topology 100 includes a floating low-noiseDVM 120 and two input terminals (here, a HI input 102 and a LO input104) that may be routed to, e.g. switched between, either the positiveinput of a gain amplifier 122 of the floating DVM 120 or to the floatingcircuit ground 121 of the floating DVM 120.

In the illustrated example, the floating DVM 120 may be electricallyconnected to the input terminals 102 and 104 with either polarity. Apre-charge or buffer amplifier 106 may be used to allow for reversal ofthe connection between the inputs 102 and 104 and the floating DVM 120without a large charge pump-out, for example. The illustrated bufferamplifier 106 has its input connected to the first input terminal 102and its supply referenced to the second input terminal 104 in order toallow either floating circuit input to be pre-charged to a voltage thatis nearly equal to the first terminal voltage prior to the firstterminal being connected to that floating circuit input.

In the illustrated example, a conductive enclosure or guard shield 101surrounds most of the DVM circuit topology 100, specifically thefloating DVM 120, and is electrically connected to the second inputterminal 104 such that there is virtually no capacitance between thefloating DVM 120 and any circuitry or component other than the secondinput terminal 104, e.g., a LO input signal. The conductive enclosure101 may be made of metal or any other suitable electrically conductivematerial.

In the illustrated example, the floating DVM 120 may have a circuitGND-based power supply 144 that is electrically coupled with a LO powersupply 140, which may also be electrically coupled with an earthGND-based power supply 142. In alternative embodiments, any of a numberof other power supply arrangements may be used so long as the supply forthe floating DVM 120 does not introduce significant coupling to earthGND circuit elements.

In the illustrated example, an analog-to-digital converter (ADC) 130 isconfigured to provide an output to an isolated communication means suchas the illustrated first opto-coupling component 132, for example. Asecond isolated communication means, such as the illustrated secondopto-coupling component 134, positioned outside the guard shield 101 mayalso be used. As with the alternative power supply arrangementsdiscussed above, alternatives to the illustrated isolated communicationmeans should focus on those that result in no more than an insignificantcoupling between the floating DVM 120 and the earth GND circuitelements.

Having described and illustrated the principles of the invention withreference to illustrated embodiments, it will be recognized that theillustrated embodiments may be modified in arrangement and detailwithout departing from such principles, and may be combined in anydesired manner. And although the foregoing discussion has focused onparticular embodiments, other configurations are contemplated. Inparticular, even though expressions such as “according to an embodimentof the invention” or the like are used herein, these phrases are meantto generally reference embodiment possibilities, and are not intended tolimit the invention to particular embodiment configurations. As usedherein, these terms may reference the same or different embodiments thatare combinable into other embodiments.

Consequently, in view of the wide variety of permutations to theembodiments described herein, this detailed description and accompanyingmaterial is intended to be illustrative only, and should not be taken aslimiting the scope of the invention. What is claimed as the invention,therefore, is all such modifications as may come within the scope andspirit of the following claims and equivalents thereto.

I claim:
 1. A system comprising: a first input terminal; a second inputterminal; a floating circuit that is physically separated from the firstand second input terminals by a conductive enclosure that has the sameelectric potential as the second input terminal, wherein the floatingcircuit includes a gain amplifier and is surrounded by the conductiveenclosure; and one or more switches within the conductive enclosure andconfigured to switch input signals received from the first and secondinput terminals between a positive input of the gain amplifier and afloating circuit ground of the floating circuit.
 2. The system of claim1, wherein the first input terminal is configured to receive a HI inputsignal, and further wherein the second input terminal is configured toreceive a LO input signal.
 3. The system of claim 1, wherein theconductive enclosure is made of metal.
 4. The system of claim 1, furthercomprising a buffer amplifier having an input connected to the firstinput terminal and a supply referenced to the second input terminal,wherein the buffer amplifier is configured to allow either of twofloating circuit inputs to be pre-charged to a voltage that is nearlyequal to the first input terminal voltage prior to the first inputterminal being connected to a first one of the two floating circuitinputs.
 5. The system of claim 1, further comprising ananalog-to-digital converter (ADC) configured to provide an output to afirst isolated communication means.
 6. The system of claim 5, whereinthe first isolated communication means includes an opto-couplingcomponent.
 7. The system of claim 5, wherein the first isolatedcommunication means is coupled with a second isolated communicationmeans.
 8. The system of claim 7, wherein the first and second isolatedcommunication means each include an opto-coupling component.
 9. Thesystem of claim 7, wherein the second isolated communication means ispositioned outside the conductive enclosure.
 10. The system of claim 1,further comprising a LO power supply configured to provide power to thefloating circuit from an earth GND-based power supply, and a circuitGND-based power supply configured to provide power to the floatingcircuitry from the output of the LO power supply.
 11. The system ofclaim 1, wherein the floating circuit is a floating digital voltmetercircuit, the system further comprising: a buffer amplifier having aninput connected to the first input terminal and a supply referenced tothe second input terminal; a LO power supply configured to provide powerto the floating circuit from an earth GND-based power supply; and acircuit GND-based power supply configured to provide power to thefloating circuitry from the output of the LO power supply.